Download MAGENTIC FIELD

Physics 42 Lab 8
The Earth’s Magnetic Field
PARTS LIST
Part
Plastic Rulers
BB Cables
Dip Angle
Power Supply
Quantity
1
3
1
1
Ammeter
1
Tangent
Galvanometer
1
FIRST: Generate Data: Pre-lab.
If you did the prelab, transfer the values you found for the Earth’s
magnetic field to this table. If you didn’t do the prelab, do it now: Find
the Earth’s magnetic field from this NOAA magnetic field calculator.
http://www.ngdc.noaa.gov/seg/geomag/jsp/IGRFGrid.jsp Use zip
code (95404 ) first to generate the latitude and longitude. Use an
elevation (.05 m) for SRJC. Record the values for th e B (given as F),
BH (given as H, and its components X(+N) and Y(+E)) and BV (given
as Z(+D). View your results in list format and enter the values in this
table.
B
D (deg)
I (deg)
H (nt)
X (nt)
Y (nt)
Z (nt)
F (nt)
Part 1. Finding the Earth’s Magnetic Field with a Tangent Galvanometer
If the direction of the horizontal component of Earth’s
magnetic field is BH and the external magnetic field
generated by the tangent galvanometer is Bi, the magnetic
needle of a compass will be directed toward BS (the total
magnetic field), as shown in the vector diagram here.
Bi
BH
θ
BS
By measuring θ and calculating the value of Bi {Bi = μ0NI/(2R)}, one can measure the horizontal
component of the Earth’s local magnetic field.
1. Measure the diameter of the galvanometer to
at least a fraction of a milimeter to at least 4
significant figures. Record your uncertainty.
2. Construct a series circuit with: the “tangent
galvanometer”, an ammeter, and the 0-6 volt
power supply. Connect the cables across the
contacts with the greatest number of loops (5
& 10) for a total of N = 15 loops. Be sure to
connect the ammeter through the unfused 20A
socket!!
3. Orient the plane of the wires so they align with the compass needle. Rotate the compass case to
zero the measuring needles (the long needles). Be patient and give the needle time to settle, and
then make fine adjustments. It is critical that the field generated by the tangent galvanometer is
perpendicular to the Earth’s magnetic field, so take care on this alignment!
4. Supply a 0.5A current and measure θ and record its value in the data sheet. Change current to 0.75A and
measure θ again. Repeat for I = 1.0 A
5. Using B = μ0NI/(2R), where N is the number of turns (15) for the wire and R is the radius of the loop,
calculate Bi and record its value in the data sheet. Keep 5 significant figures (for the heck of it) and
express your answers in nT so you can compare your values to the data.
6. Using the angles from your data, calculate each BH and then calculate an average value.
7. Using the dip needle and assuming Elliot Ave is geographic due north, find the Declination and
Inclination angles of the Earth’s local magnetic field.
8. Using the dip angle and the average value of the horizontal component of the Earth’s magnetic field,
calculate values for the vertical component and total magnetic field of the Earth. Show all your
calculations in the provided space. Keep 5 significant figures (for the heck of it)
9. Compare your measured and calculated values to the NOAA values with percent differences. Briefly
discuss your results and possible sources of error. Please type and print it out.
Earth’s Magnetic Field Data Sheet
Diameter and radius of circular loop including uncertainty:
d (m) = __________________________
I (A)
0.5
θ (degrees)
r (m) = ____________________________
Bi (nT)
BH (nT)
B
0.75
1.0
Average Be from measurement (nT) =
Quantity
BH (nT)
Measured Value
B
Declination
(degree)
Inclination
(degree)
Bv (nT)
B (nT)
Show Calculations of Bv and B:
NOAA Value
% Difference
Part 2. The Earth’s Magnetic Field and Pole Reversal
Go to the website for the NOAA Geomagnetism and check it out:
http://www.ngdc.noaa.gov/seg/geomag/geomag.shtml
Read the FAQ on the Earth’s magnetic field to answer the questions below.
http://www.ngdc.noaa.gov/seg/geomag/faqgeom.shtml
a.
What causes the Earth’s Magnetic Field?
b. What is the dynamo effect?
c. Where is the Magnetic North Pole now?
d. How fast is it moving? What direction is it moving in?
e. How do we know the poles are changing and that one day they may flip?
f. How do we know the poles flipped in the past?